In many of today's applications large amounts of data need to be stored. In the area of Personal Computers and in addition in the area of consumer electronics harddisks as fixed storage media and optical disks as removable storage media are commonly used for this purpose. Due to emerging incremental data bandwidth needs (HDTV, fast copy) the data throughput of the applications is increasing continuously. Consequently, the corresponding disk devices have to provide the appropriate data and data bandwidth, too. Therefore, a plurality of high density optical recording media have been developed, e.g. the Digital Versatile Disk (DVD) or the Blu-Ray Disk (BD) using blue laser light for reading and recording.
The readout signal from such high density optical disks is deteriorated mainly by two factors. The first factor is the small ratio between the amplitudes of the shortest run-length symbols and the longest run-length symbols (e.g. the I2/I8 ratio in the case of Blu-Ray disks), which is caused by the high information density, i.e. the ratio between the diameter of the laser spot and the physical length of a channel bit. This ratio is time-variant due to effects like disk tilt or de-focusing. The second factor is the waveform asymmetry, which is caused by an unequal length of the pits and lands due to recording and/or mastering imperfections. This asymmetry is also known as domain bloom.
In “Combined Adaptive Controlled Partial Response and Maximum Likelihood Signal Processing for High-Density Optical Disks”, Jpn. J. Appl. Phys. Vol. 42 (2003), pp. 924-930, Takehara et al. propose to use both an adaptive equalizer and a Viterbi detector with adaptive reference levels in order to allow a highly reliable recovery of the stored user data. However, both adaptation schemes rely on the recovered bit stream and are in fact gain control loops. Therefore, the adaptation is difficult to control and tends to instability. Furthermore, this detector arrangement has problems at start-up, i.e. it needs a proper pre-setting of all coefficients and a complicated range checking during the adaptation process. Finally, an additional automatic gain control loop, which is also necessarily present, forms a third loop which modifies the gain.
It is an object of the invention to propose an improved method for adaptive bit recovery.